ObjectiveThe aim of this study was to investigate the short-term and long-term speech effects of the Lee Silverman Voice Treatment (LSVT) program in a group of individuals with flaccid dysarthria and compare its effectiveness with traditional dysarthria therapy (TDT) methods.DesignThis study was designed as a randomized controlled trial.Patients and methodsThe study included 22 patients with flaccid dysarthria who were randomized into two groups and received either the LSVT or the TDT. Both interventions were administered at the same frequency, for 1 h a day, 4 days a week, for 4 weeks. Participants underwent a three-phase assessment: (a) before treatment, (b) immediately after treatment, and (c) 6 months after treatment or follow-up. Outcome measures included auditory perceptual analysis, acoustic and spectral analysis, aerodynamic analysis, nasometry, and Communication Partner questionnaire.ResultsThere was significant improvement in most auditory perceptual analysis and instrumental variables in both treatment groups but most of these improvements were not maintained during follow-up in either group. Comparative statistical analysis revealed no significant differences between the two groups in the three assessment phases in most studied variables.ConclusionThe apparent equity between LSVT and TDT in flaccid dysarthria highlights the clinical value of teaching the LSVT method and the potential for this treatment to yield improvements comparable to traditional articulation therapy methods through a technique that is cognitively less demanding. Thus the advantage of LSVT is not the amount of improvement but the less-demanding nature, cognitive-wise, of the technique.

Flaccid dysarthria is a perceptually distinguishable type of dysarthria resulting from impairment of the lower motor neurons of the cranial or spinal nerves. Its characteristics reflect the combined effect of weakness and reduced muscle tone on the speed, range, and accuracy of speech movements, hence its designation as flaccid dysarthria [1],[2] . The effect of dysarthria on communication can be chronic and debilitating and has the potential to significantly affect an individual's quality of life [3] . An extensive range of traditional treatments for dysarthria are available and widely used in the clinical setting [4-6]. Behavioral management, which aims to maximize communication efficiency, naturalness, and intelligibility through direct symptom treatments and compensatory strategies [2] , is considered the most common approach in dysarthria [1] . Despite the wide use of traditional articulation strategies, there exists limited research exploring the efficacy of these techniques in nonprogressive dysarthria. A recent Cochrane review concluded that there is currently a paucity of rigorous scientific evidence supporting the efficacy of treatments that are typically used for this population [7] .

The Lee Silverman Voice Treatment (LSVT) program began as a treatment program designed and validated for individuals with progressive hypokinetic dysarthria due to Parkinson's disease (PD) to increase vocal volume [8] . Research has revealed the value of the program in improving articulation in this population, and thus it now includes other neurological disorders. The LSVT is administered intensively for 1 h a day, 4 days a week, over 4 weeks, and continually focuses on increased loudness [8] . Studies have revealed that increased loudness results in global improvement across the speech mechanism in individuals with PD [9],[10],[11],[12] . Moreover, as the speaker is required to remember only one concept (loud) during therapy, the automaticity of his or her speech is not hindered [13] and can compensate for the cognitive and psychosocial deficits in the traumatic brain injury (TBI) and stroke population.

The LSVT can modify or compensate for underlying respiratory-phonatory insufficiency, which is an important pathophysiological change in flaccid dysarthria [1] . The loud and effortful phonatory tasks of the LSVT are aimed at improving respiratory drive, vocal fold adduction, laryngeal muscle activity and synergy, laryngeal and supralaryngeal articulatory movements, and vocal tract configuration. These physiological changes should improve voice quality and loudness, articulatory precision, prosodic inflection, resonance, and speech intelligibility [14],[15] .

Short-term and long-term improvements to phonation, articulation, and intelligibility have been found in individuals with nonprogressive dysarthria following LSVT [16],[17] . Although the results of these studies provided promising evidence of the effects of LSVT on articulation in nonprogressive dysarthria, further investigation that incorporates both perceptual and objective measures (e.g. acoustic analyses, aerodynamic measures, nasometer) to monitor intervention effects on articulation would help substantiate findings.

Wenke et al.[18] revealed that LSVT may be a suitable treatment option for improving vowel articulation and subsequent intelligibility in some individuals with nonprogressive dysarthria, but the authors could not give definitive conclusions as they evaluated diverse types of dysarthria patients and they directed the study of specific types of dysarthria for further trials.

Thus, the global effect of therapy on all aspects of speech, including phonation, respiration, resonance, articulation, and prosody, in patients having dysarthria - including the flaccid type - still warrants investigations.

Objectives

The aims of our study were, first, to explore the effects of LSVT on all aspects of speech, including phonation, respiration, resonance, articulation, prosody, and intelligibility, in a group of individuals with flaccid dysarthria. Our second aim was to compare the effectiveness of the LSVT program with traditional dysarthria therapy (TDT) methods using the same parameters.

Patients and methods

The study included 22 patients with flaccid dysarthria who were randomized into two groups, each group including 11 patients. Inclusion criteria were a diagnosis of flaccid dysarthria according to the original Duffy 1995 classification [1] , resulting from stroke or TBI, occurring not less than 6 months previously to minimize the effects of spontaneous neurological recovery on the participants' speech function. Participants were required to have no additional diagnoses of dementia or aphasia, and to have adequate hearing and vision.

The exclusion criteria included patients with dementia, aphasia, or hearing and visual impairment. Patients with mixed-type dysarthria or dysarthria of less than 6 months' duration were also excluded.

Participants underwent a three-phase assessment:

before treatment,

immediately after treatment, and

6 months after treatment or follow-up.

Both groups were assessed using the protocol of dysarthria assessment, as follows.

Auditory perceptual analysis of speech

The auditory perceptual analysis (APA) of speech was carried out using high-fidelity voice recording using a SONY recorder, model VACS, with a microphone (IMP 600) (Sony Corporation, Japan). The speech task included sustained vowel prolongation, syllable repetition, standard passage, and spontaneous speech samples. The judges used speech records to comment on the degree of speech imprecision, nasality, dysphonia severity, and overall dysarthria severity. Speech rate was measured from the sample of connected speech devoid of pauses as word per minute. Two intelligibility measures were calculated: word intelligibility percent and utterance intelligibility percent. Three professional judges transcripted the sentences word by word and the average number of intelligible and total words and utterances were calculated from the transcription sheet of all judges [19] .

Acoustic and spectral analysis

Acoustic and spectral analysis of the patient speech was carried out using Kay CSL, model 4500. The patient sits comfortably at a 10 cm distance from a microphone, repeating the standard sentence (Tami wi sami bijbo Kora fil fosha). The data are obtained by visual calculation using horizontal and vertical cursors on the screen. It includes voice-onset time (VOT) for /t/ and /k/, formant transition for vowel /u/ and F1, F2 for the three vowels /a/, /i/, and /u/.

It was carried out using a Kay Nasometer, model 6200, to measure the nasalance score of oral and nasal sentences.

Communication Partner questionnaire

This questionnaire required each participant's primary communication partner to rate five different aspects of the participant's everyday conversational abilities on a scale of 1-7, with a rating of 7 indicating very good ability and a rating of 1 indicating very poor ability. Questions included the following:

How easy is it to understand the speaker?

How often do you ask the speaker to repeat themselves?

How often does the speaker initiate conversation with you?

How often does the speaker initiate conversation with an unfamiliar person? and

Individuals received either the LSVT or TDT following the outcome of the randomization procedure. Both interventions were administered at the same frequency, 1 h a day, 4 days a week, for 4 weeks (in addition to homework exercises). All treatment sessions were administered by the same phoniatrician, with all participants completing the prescribed 16 sessions within ~4 weeks.

Lee Silverman Voice Treatment

The LSVT was administered in strict accordance with the manual by a phoniatrician experienced in delivering the program. The LSVT program focused on the participant constantly employing increased vocal loudness and optimal physiological effort. Each treatment session consisted of the participant performing multiple repetitions of the following tasks using a high-effort, loud, healthy-quality voice:

(a) maximum duration of sustained vowel phonation (i.e. /a/);

(b) generating the highest and lowest pitch levels the participant could achieve; and

(c) reading a list of functional phrases.

These tasks were followed by successively more complex speech tasks during which the participant used the same loud voice as taught in the repetitive drills. As part of the program, each participant was also required to complete daily homework multiple repetition tasks at least three times a day, for 10 min each, which were revised at the next session to check that the patients had consistently and regularly followed through at home.

Traditional dysarthria therapy

From the audio recordings of the initial assessments, the authors drew up behavioral intervention aims and approaches for each participant. Speech therapy programs were adopted in the form of 1 h individual sessions, four times weekly, for 4 weeks. The speech characteristic of each patient was classified pathophysiologically into clusters and each cluster was dealt with according to the speech component affected [1] . Consequently, in addition to targeting articulation, therapy targeted the respiratory/phonatory system, as well as speech processes such as resonance and prosody that had been identified in the pretreatment assessment as contributing substantially to reduced speech intelligibility. The traditional management strategies employed were behavioral techniques and they consisted of four to five different activities in each session. Activities that were drilled (e.g. sustained phonation, oromotor exercises) involved approximately six to seven repetitions for each task. For all participants the overall goal was the maximization of comprehensibility and effectiveness in conversational speech.

The treatment intensity, high effort, clinician feedback, daily homework, daily quantification of treatment variables, and carryover were all presented and stimulated equally in both treatment groups. Participants in both groups were also provided with maintenance exercises to be undertaken independently following the cessation of treatment for 5-10 min a day, 3-5 days a week, for 6 months.

Ethics

The Institutional Review Board (IRB) at Dubai Hospital approved the protocol of this study, and written informed consent was obtained.

Statistical analysis

Statistical analysis was performed using SPSS, version 20 (SPSS Inc., Chicago, Illinois, USA). Independent t-tests between the two groups were conducted for all variables at baseline, which revealed no significant difference between groups before treatment. Significant differences between pretreatment and post-treatment means for the group and pretreatment and follow-up means for the group's perceptual and acoustic data were calculated using the paired t-test, one-way repeated-measures analysis of variance, and the χ2 -test for numerical and rank variables.

Results

Auditory perceptual analysis

The APA revealed significant improvement in the degree of articulatory imprecision, overall dysarthria severity, and word intelligibility in the LSVT group. However, these changes were not maintained during follow-up. The improvement in the TDT group was significant in terms of degree of articulatory imprecision, hypernasality, and overall dysarthria severity, with significant increase in speech rate and word and utterance intelligibility. With the exception of articulatory precision and word intelligibility, these changes were not maintained during follow-up ([Figure 1], [Figure 2], [Figure 3] and [Figure 4]).

[Table 1], [Table 2] and [Table 3] show the pretherapy, post-therapy, and follow-up numerical outcome measures of the LSVT group and the TDT group. Acoustic measures revealed that the LSVT resulted in significant change in formant transition, F1 and F2 for vowels /a/ and /u/. There was a tendency for increased F1 and decreased F2 for both vowels /a/ and /u/. The VOT for the studied consonants /t/ and /k/ had reduced but did not reach significance. TDT resulted in significant reduction in formant transition for vowel /u/, F1 for vowels /a/ and /u/, and VOT for /t/ and /k/. F2 for vowels /a/ and /u/ had significantly increased, which was the same trend in LSVT, but these changes were not significant for vowel /e/. All spectral changes failed to be maintained during follow-up in both groups, except that VOT for /t/ was maintained significantly lower in the TRD group. Aerodynamic measures revealed significant improvements in all measured parameters in both groups regarding phonatory SPL, VC, MPT, PQ, and Psub. During follow-up, phonatory SPL was maintained significant in both groups, whereas Psub was maintained significant only in the TDT group. There was significant reduction in oral sentence nasalance score in both groups after therapy measure but it failed to be maintained during follow-up.

Table 1 Pretherapy, post-therapy, and follow-up numerical outcome measures in the LSVT group

Comparative statistical analysis revealed no significant differences between groups in the three assessment phases in most studied variables except in VOT for /t/, which was significantly lower in the TDT group after therapy and during follow-up; also Psub was significantly higher in the TDR group during follow-up.

The Communication Partner questionnaire revealed significant improvement in the rate of understanding the speaker (Z = −2.23, P = 0.026), the speaker initiation of conversation (Z = −2.29, P = 0.019), and overall rating of the speaker's speech and voice (Z = −2.12, P = 0.034) in the LSVT group. However, these changes were not maintained during follow-up. The improvement in the TDT group was significant for the same questions, and these changes were not maintained during follow-up except for the rate of understanding the speaker (Z = −2.27, P = 0.021).

Discussion

Flaccid dysarthria is a common sequel of nonprogressive brain damage (typically stroke and traumatic brain damage). Impairment-based therapy and a wide variety of techniques for the treatment of flaccid dysarthria have been studied. The common aim of the treatment is to contribute to increasing intelligibility or communication effectiveness [20] .

The current study investigated the effect of LSVT in individuals with flaccid dysarthria and compared it with traditional therapy. The outcome measures reflected perceptually and instrumentally the changes in all aspect of speech, including respiration, phonation, articulation, resonance, prosody, and overall speech intelligibility, in the LSVT group.

Auditory perceptual analysis

The improved articulatory precision and word intelligibility in the LSVT group in the current study could be related to larger amplitudes of articulatory movements that result in more accurate articulatory contacts. Because of the strong correlation that exists between articulation and intelligibility in dysarthric speakers [21] , the perceived improvement in articulatory precision may have further contributed to the improved speech intelligibility and overall dysarthria severity demonstrated in this group following LSVT. Short-term and long-term improvements to articulatory precision have also been found in a study by Wenke et al.[16] and Sapir et al.[17] using the LSVT program. The authors speculated that perceived improvements were related to the global effect that increased loudness has on articulatory function, with prior research associating loudness with increased amplitude of articulatory movements [22],[23] .

TDT resulted in similar improvement to LSVT, with more significant improvement in the degree of hypernasality, speech rate, and utterance intelligibility. These improvements were most likely an effect of the articulation exercises, intelligibility drill, rate control, and velar exercises that were incorporated as part of the TRAD program. This APA improvement in dysarthric speakers in both groups explains the improvement in the Communication Partner questionnaire scores.

The improved rate in the TDT group may be due to fast articulatory precision and improved breath control used as part of prosodic treatment in TDT, which also improved both words and utterance intelligibility in that group more than what was observed in the LSVT group as the prosodic aspect of speech was not directly trained in this group.

The impact of LSVT on flaccid dysarthria was not truly global, as remaining deficits to resonance and prosody subsystems may explain unchanged ratings on some outcome measures.

Acoustic measures

The reduced formant transition noted after LSVT in the present study could be explained by the effect of increased loudness on the velocity of articulatory movements. Research has revealed strong correlations between the velocity of lip and jaw movements and loudness in dysarthric speakers [13] . Vowel formants changes in both groups may suggest that loud speech, as trained in the LSVT method, may also influence tongue movement in dysarthric speakers. The trends of decreased F2 values for /a/ and /u/ may reflect greater tongue retraction [24] , and the trend of increased F1 values found following LSVT might also indicate reduced tongue height, resulting in a more lowered tongue position [25] . This lower more retracted tongue position may have resulted in an increased size of the anterior oral cavity and subsequently greater vowel space area. Wenke et al.[18] found a similar trend in vowel formants but the finding was not significant because of variability of dysarthria types and the heterogeneous nature of articulatory impairment with high intersubject variability. Additional explanations for vowel formant changes are increased lip rounding, lip movement velocity, and jaw displacement [26] .

Acoustic measures also supported the emergent trend that TDT therapy improved articulatory movements with significant changes in formant transition, F1 and F2 values, and VOT for /t/ and /k/. These improvements were most likely an effect of the articulation exercises that were incorporated as a part of the TDT program. These temporal changes also support the improved rate in the TDR group.

Aerodynamic measures

Respiratory-phonatory insufficiency is a common feature of flaccid dysarthria and can manifest as reduced loudness, Psub, VC, and MPT [27] . In the current study, the LSVT was found to improve all aerodynamic measures, which reflected a degree of improvement in respiratory-phonatory function. Similar improvements have been reported in previous studies [16, 28, 29]. Research has shown increased loudness to be associated with greater inspiratory lung volumes in normal speakers and increased adduction of the vocal folds in individuals with PD [15, 30, 31]. Similar changes were noted in the TDT group and can be attributed to direct respiratory-phonatory exercises with improved vocal fold adduction and respiratory-phonatory control.

Resonance

The improved resonance in the TDT group was expected as it is directly targeted in the TDT therapy program, as successful management of velar function will not only improve resonance but will also enhance other subsystems. Thus, Dworkin [32] states that resonance should receive top priority in the intervention hierarchy.

Although the LSVT did not directly target nasal resonance, the trends of reduced resonance may suggest that the increased effort and loudness as taught in the LSVT may have generated greater recruitment of motor units needed for more complete velopharyngeal closure [33] . The reduced resonance may alternatively be explained by the increased opening of the oral cavity that occurs with increased loudness. Wenke and colleagues found in their study that three out of five participants demonstrated reductions in perceived hypernasality following LSVT, and limited changes in TDT. These findings contradict with the results of the TDT group in the present study. Because of the small sample size and variability between participants, they concluded that the application of LSVT in the treatment of VPI must be trialed on an individual basis until additional research data are available [34] .

There was a mismatch between nasalance score and perceived rating of hypernasality. Similar mismatches have been found in previous studies on dysarthric speakers [33],[34],[35] . Perceptual nasality judgment can be influenced by a number of factors including speech intelligibility, articulatory precision, and speaking rate, making reliable judgment of nasality often difficult [34] This mismatch was not present in TDT as it directly targets other aspects affecting nasality perception, such as articulatory precision, speech intelligibility, and speaking rate.

Therapy maintenance

The lack of maintenance of certain variables at follow-up may be due to reduced participant compliance in performing ongoing maintenance exercises following the cessation of treatment [7] Another possible explanation may be related to the design of the LSVT and its effect on retention/transfer of targeted behaviors. The LSVT is simple and focuses on a loud, intensive practice schedule and is consistent with the principles of motor learning [36],[37],[38],[39] . However, the program's blocked practice schedule (i.e. targets practiced in successive systematic blocks) may facilitate acquisition of a behavior but not retention/transfer as opposed to random practice schedules (i.e. when treatment targets are intermixed) [40] . This information may indicate why improvement in certain parameters that occurred immediately after treatment (i.e. acquisition phase) failed to show long-term retention in the present study. Furthermore, blocked treatment schedules have been found to be less effective in the transfer or generalization of a behavior [41] . This may potentially explain why tasks that were trained for during LSVT in the current study (e.g. SPL) generally showed more consistent maintenance compared with stimuli that were not directly targeted (e.g. word and sentence intelligibility).

Effect of TDR in dysarthria and comparison with LSVT

The present study revealed similar improvement in the two groups for most of the acoustic or perceptual and aerodynamic measures of speech. Wenke et al.[42] demonstrated that the LSVT as a treatment option for individuals with nonprogressive dysarthria resulted in comparable effects to intensive TDT. Clinicians have to consider other factors co-occurring with dysarthria, such as cognition when selecting the most suitable treatment. Impaired cognitive functioning is a prevalent feature exhibited by individuals with TBI and stroke [43],[44],[45] and can result in reduced memory, new learning, and attention. It would, therefore, appear that a treatment modality that is cognitively less challenging for this population and has the potential to address multiple components of speech production would be beneficial for learning. While traditional methods of articulation therapy involve participants having to explicitly concentrate on correct placement of the articulators, individuals receiving LSVT are only required to 'go loud'. The effects demonstrate that therapy with one single focus, as increased loudness, can have a positive effect across the speech mechanism without direct impact on other systems, which may be related to the reduced cognitive demand that is required to perform this strategy as opposed to traditional articulation strategies [13] .

Conclusion

The apparent equity between LSVT and TRT with regard to global speech improvement in flaccid dysarthria highlights the clinical value of teaching the LSVT and the potential for this treatment to yield improvements comparable to traditional articulation therapy methods through a technique that is cognitively less demanding [18] .